Halo implantation is used in CMOS fabrication to suppress punch-through effect, for example. In conventional processes, low energy, low current implantation is carried out at large incident angle so that implanted dopants penetrate underneath the edge of the gate stack. However, the incident angle has geometric restrictions as the devices are scaled. This is due mainly to the pitch between adjacent devices, and the need for a thick photoresist to block the implant from penetrating into undesirable locations of the device.
For example, in conventional fabrication processes, a photoresist is deposited over the structure in order to block penetration of the halo implant into undesirable locations of the device. For this reason, the photoresist must be perfectly aligned with an edge of a gate structure. However, to adequately protect the device with a photoresist, it is necessary to deposit a very thick layer of photoresist, typically on the order of 0.5 to 3 microns thick. This thickness, however, poses many problems.
For example, a thick photoresist causes reflection of the incident beam into an undesirable exposed portion of the device, which becomes even more pronounced when the node shrinks (i.e., the pitch between adjacent devices is small). That is, at smaller nodes, the rate of reflection becomes even more problematic, as the resist is very thick and the implant can very easily bounce off the walls of the resist into undesirable locations of the device.
Also, due to the thick photoresist, it necessary to provide a very large opening in the resist in order to implant the dopant at the required high incident angle. Again, this becomes problematic in smaller nodes in that it is not possible to provide a large enough opening in the resist due to the spacing between adjacent devices. For example, in smaller nodes, the pitch between the devices becomes ever more smaller, which restricts the size of the opening in the resist. For this reason, it may not be possible to provide a large opening needed to perform the high incident angle halo implant. Also, at these smaller nodes, it becomes more difficult, time consuming and costly to properly align the resist with the edge of the device.
Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.